The invention relates to an apparatus for the total reflection X-ray fluorescence analysis (TRFA) in which the smooth planar surface of a sample or thin film on a sample is excited by the incident X-ray radiation and the X-ray fluorescence radiation emitted is detected spectrally and which comprises a radiation source, a monochromator and a transducer. The invention further relates to the use of such an apparatus for the determination of foreign atom concentrations on wafer surfaces and to a method for the high-sensitivity resolution measurement of foreign atom concentrations on a wafer.
In total reflection X-ray fluorescence analysis, the surface of a sample is exposed at a very flat angle of incidence to the X-ray radiation generated by an X-ray tube. The angle of incidence is selected so that the incident X-ray is totally reflected. The total reflection geometry means that there is only a certain radiation intensity in a near-surface layer of around 3 nm thickness. For this reason, only the atoms of this thin surface area is excited by the incident X-ray radiation. The excited atoms emit fluorescent X-ray quanta of a certain energy which are characteristic for the relevant atom. The energy spectrum emitted by all surface atoms is measured using a detector and the concentration of an element in the thin surface film can be determined from the intensity of the peak associated with the respective element.
Germanium or silicon detector (Si(Li)) are used to register the energy spectrum with the apparatus for X-ray fluorescence analysis used up to now. All these detectors utilize the fact that the incident X-ray quanta have a very strong ionizing effect and so generate a plurality of charge carriers in the semi-conductor material. The higher the energy of the X-ray quantum, the more charge carriers are generated. The charge quantities generated in the detector are therefore measured at periodic intervals and transduced into an energy spectrum by means of a multi-channel analyzer to read out the detector. Counts of up to 2-104 counts per second (cps) can be achieved with such detectors; 2-104 X-ray quanta per second can therefore be measured.
One disadvantage of such detectors is that they only supply high-resolution energy spectrums when the thermal noise is effectively suppressed by cooling with liquid nitrogen. The required nitrogen Dewar flasks are large and problematic to handle.
German Patent DE196 20 081 A1 and PCT application PCT/DE97/01015 describe a strip detector and a method of manufacturing a strip detector for the detection of ionizing particles and/or radiation. Regions n-doped at least at one substrate surface of the silicone substrate and a p-doped insulation region between the n-doped regions are provided as strips and voltage supply areas. A first insulator film is applied to one substrate surface and metal strips are arranged above the n-doped region.
The invention is characterized by at least one further insulation film being provided immediately above the first insulator film and by at least one of the insulation films being interrupted in projection over the intermediate region of two adjacent n-doped regions and by the p-doped insulation area having a lateral p-dope material concentration distribution which provides a higher dope material concentration in the region beneath the interruption of the interrupted insulation film than in the insulation region directly adjacent to the n-doped region. The insulation structure in accordance with the invention is also suitable for the insulation of guard rings which gradually reduce the high operating voltage towards the detector periphery. It insulates adjacent rings extending around the whole detector so that different potentials with low electrical field strengths can develop on them.
It is the object of the invention to provide an apparatus and a method for X-ray fluorescence analysis to allow a faster measurement with better resolution of the X-ray fluorescence of a sample and a reduction in the effort required to cool the detector.
This object is solved in accordance with the invention by an apparatus for total reflection X-ray fluorescence analysis in which the smooth planar surface of a sample or thin film on a sample is excited by the incident X-ray radiation and the X-ray fluorescence radiation emitted is detected spectrally and which comprises a radiation source, a monochromator and a transducer, with the transducer comprising at least one DRIFT detector, an electrical field being capable of generation in each DRIFT detector by means of an electrode array of electrodes at different voltage levels and having a radial component. Charge carriers which are created are accelerated towards a low-capacitance collecting electrode by this radial component of the electrical field.
Counts of up to 105 cps can be achieved by using a transducer with at least one DRIFT detector. This is made possible by the charge carriers generated by an X-ray quantum being guided faster to the collecting electrode through the electrical field prevailing in the detector interior. The main reason for the high count capable of being achieved with the aid of DRIFT detectors is, however, the low capacitance of the collecting electrode. The radial component of the electrical field accelerates the charge carriers towards the collecting electrode and as a result the collecting electrode can be made with a small surface and so a low capacitance.
In summary it can be said that the read-out times can be shortened significantly when DRIFT detectors are used. Higher counts can be processed in this way and spectrums with better energy resolution are obtained. DRIFT detectors also require less effort for cooling; in particular, the high-effort nitrogen cooling can be omitted.
It is of advantage if the electrode array comprises a plurality of annular electrodes arranged concentrically and at different voltage levels. With such an electrode array, an electrical field with a radial component is produced.
In accordance with another advantageous embodiment of the invention, a first transistor of a pulse amplification stage is integrated in the collecting electrode. This can be done by the film structures of a field-effect transistor being integrated in the centre of the DRIFT detector, the gate of the field-effect transistor being electrically connected to the collecting electrode. In this way, the connection between the collecting electrode and the gate of the FET can be shortened and the parasitic capacitance of the connecting line is greatly reduced. Moreover, there is a reduction in the noise captured by the inductive coupling.
In accordance with another aspect of the invention, the DRIFT detector can be cooled by means of a Peltier cooler element. A cooling of up to 30xc2x0 relative ambient can be achieved with a one-stage Peltier cooler element. Such a cooling is sufficient to allow highly resolved energy spectrums to be registered by DRIFT detectors. The nitrogen cooling required for prior detectors can therefore be replaced by a Peltier cooler, with the Peltier element being integrated directly in the DRIFT detector.
It is of advantage to arrange the radiation source, the monochromator and the transducer in a vacuum housing. Moreover, it is of advantage if the transducer has a thin window of a thickness of less than 2 xcexcm as the measurement window. It is furthermore of advantage if the radiation source consists of a low energy X-ray tube.
The measurement range of total reflection X-ray fluorescence analysis can be expanded to elements with a low ordinal number Z, that is to light elements, using the three above-mentioned measures. These elements emit fluorescence X-ray radiation of only low energy. The unwanted absorption values of the X-ray radiation can be reduced with the aid of the vacuum housing and of the thin measurement window. A low-energy X-ray tube emits excitement radiation which is near the energy of the fluorescent X-ray radiation of light elements. The fluorescence efficiency can be increased in this way.
It is of advantage to arrange a sample table opposite the transducer to accommodate a sample, with the sample table being made longitudinally displaceable preferably in the z direction and the x,y direction, rotatable around its middle axis and pivotable in a circle segment guide.
In total reflection X-ray fluorescence analysis, the angle of incidence has to be set precisely to 0.1 to 0.2 arc minutes. This can be done using an adjustable precision sample table.
It is furthermore of advantage when the sample table has a suction plate as the accommodation plate in which a plurality of pints are integrated which can be travelled above its upper plane. With the help of the suction plate, the sample, for example a wafer, can be sucked in the direction of the sample table. A more precise guide and a more exact positioning of the sample can be achieved in this way. To detach the sample or wafer from the plate again, pins provided for this purpose are travelled out. The sample or wafer can then be removed, for example, by a robot grabber.
In accordance with another advantageous embodiment of the invention, a scattered radiation diaphragm is positioned in front of the transducer. Such a scattered radiation diaphragm prevents scattered X-ray radiation from becoming incident on the transducer by the scattered radiation being absorbed by the diaphragm.
It is of advantage here if the scattered radiation diaphragm can be travelled in a z direction, for in this way, an adaptation of the position of the scattered radiation diaphragm to the thickness of the sample measured in each case can be carried out.
In accordance with another advantageous aspect of the invention, the excitation of the sample occurs by means of an incident parallel X-ray which is generated with the aid of parabolically arranged Bragg reflectors which reflect the X-ray radiation generated by the X-ray tube. Exactly as with a parabolic mirror, it is possible with this arrangement of Bragg reflectors to form a parallel X-ray from the divergent radiation generated by the X-ray tube. By the X-rays emitted in different directions being capable of bundling into one ray, an excitation ray of high intensity can be provided. Either crystals or multi-layer systems can be used as the Bragg reflectors.
In accordance with another advantageous embodiment of the invention, the transducer has a detector array of a plurality of DRIFT detectors which allows the high-sensitivity resolution recording of the X-ray fluorescence radiation of the sample. A statement can be made on the distribution of the different elements on the sample surface by means of such an array of DRIFT detectors. For this purpose, each of the DRIFT detectors may only detect the fluorescence radiation from a defined small region of the sample. A low fluorescence intensity can be utilised better with the aid of DRIFT detectors than with conventional detectors and good energy spectrums are achieved despite the low fluorescence intensity. Only in this way is it possible to achieve a high-sensitivity resolution recording.
However, for this purpose, it is necessary for each DRIFT detector to have its own pulse amplification chain. It can be expected that in the near future integrated circuits will be available in which the required pulse amplification electronics is integrated.
When an array of DRIFT detectors is used, it is of advantage if the detector array can be operated in a first and a second mode, with the X-ray fluorescence radiation of the probe being measured at high-sensitivity resolution in the first mode and the sum spectrum of the X-ray fluorescence spectrums supplied by the individual detectors being determined in the second mode.
While the high-sensitivity resolution measurement allows a statement on the distribution of the elements on the surface measured, the sum spectrum allows a lower resolution limit because a substantially higher number of fluorescence X-ray quanta was taken into account in the calculation of the sum spectrum. The evaluation of the sum spectrum allows a statement on the mean concentrations of foreign atoms on the sample or wafer surface.
In accordance with another advantageous aspect of the invention, the sample and the transducer can be moved relative to one another in the x,y direction. Here, the transducer comprises a detector array of a plurality of DRIFT detectors. It is possible in this way to detect the foreign atom concentrations of a wide-area sample, in particular of a wafer, with a detector array only covering a part of the wafer surface. Here, the detector can be moved over the stationary sample, on the one hand, and a stationary detector array can be provided, on the other hand, under which the sample is moved. A third possibility is to move both the detector and the sample.
With regard to a high-sensitivity resolution recording of the X-ray fluorescence radiation, it is in particular of advantage in the measurement of circular samples to make the detector array in the form of at least one circle detector, with the detector array and sample being pivoted relative to one another with respect to an axis extending through the sector tip. The detector array consisting of a plurality of DRIFT detectors therefore covers at least one sector in each case of the circular sample or wafer to be measured and a high-sensitivity resolution detection of the fluorescence spectrums is then performed with respect to this part of the sample. Subsequently, the sample or wafer is turned an increment, e.g., one xe2x80x9ccake wedge or slicexe2x80x9d further relative to the detector to record respective fluorescence spectrums.
When a detector array comprising two diametrically opposed circle sectors is used, the additional advantage of a shorter measuring period results.
In accordance with another advantageous embodiment, the detector array covers an area which is greater than the surface of the sample. In this way, the high-sensitivity resolution recording of the X-ray fluorescence of a sample can be performed with a minimum measuring time.
The apparatus in accordance with the invention for the total reflection X-ray fluorescence analysis can, in particular, be used to determine foreign atom concentrations on wafer surfaces. The silicon wafers used for semi-conductor production are subject to the highest demands for the purity of the silicone used. There are tightly limiting tolerance regions which have to be monitored with regard to foreign atom concentrations. Total reflection X-ray fluorescence analysis is suitable for this purpose in that conclusions with respect to the foreign atom concentrations can be made immediately from the fluorescence spectrums by means of a suitable calibration. The measuring precision can be further increased by the use in accordance with the invention of DRIFT detectors, whereby even very low foreign atom concentrations can be detectable in quantity. A high-sensitivity resolution determination of the foreign atom concentrations on the wafer surface allows a statement on the distribution of different elements on the wafer surface and is therefore of help in being able to find the cause of contaminations. It is important that the apparatus in accordance with the invention for total reflection fluorescence analysis can be integrated in a semi-conductor production line easily and therefore allows a constant quality monitoring.
In the method in accordance with the invention for the high-sensitivity resolution measurement of foreign atom concentrations on a wafer when using a detector array comprising a plurality of detectors, the following steps are performed: first, a relative movement is generated between the detector array and the sample to be measured, wherein the respective local X-ray fluorescence spectrums are transduced. Subsequently, the respective topical foreign atom concentration is calculated from the respective X-ray fluorescence spectrums recorded there.
In this way, the total wafer surface can be checked as regards its content of foreign atoms, with the detector array not having to cover the whole wafer area. The concentrations of foreign atoms at a certain point here result from the size of the peak in the spectrum recorded there. As a result, a high-sensitivity resolution representation of the concentrations of all foreign atoms can be obtained simultaneously with the measurement method described. The method in accordance with the invention for the measurement of the mean foreign atom concentrations on a wafer while using a detector array comprising a plurality of detectors comprises the following steps: first a relative movement is generated between the detector array and the sample to be measured and the relative local X-ray fluorescence spectrums are recorded. Subsequently, the topical X-ray fluorescence spectrums are summed. The mean foreign atom concentrations can then be determined from the sum spectrum obtained in this way.
The concentrations of the foreign atoms result from the peaks in the sum spectrum. The foreign atom concentrations meaned over the wafer surface can be determined with high sensitivity in a short time with the measurement method described.